This invention relates to the high temperature stabilization of aluminum titanate and aluminum titanate-mullite compositions by the addition of iron oxide. It has been discovered that iron oxide concentrations greater than 5% and as high as approximately 25% by weight have a stabilization effect at high temperatures on aluminum titanate. This effect is uncommon to prior stabilization attempts.
Aluminum titanates may be effectively used as filters for fluids, in particular, as diesel particulate filters and as substrates for catalytic converters, an example of which is known commonly in the art as a honeycomb substrate. Additionally, aluminum titanates are desirable in applications where the thermal shock resistance and the ultimate use temperature are high. Cellular substrates used under conditions of high thermal gradients are examples of this application. Typically, structures such as the above are subjected to harsh environments which require high thermal shock resistance, low thermal expansion, and high mechanical shock properties. Skilled workers in the art appreciate that maintaining these properties for extended periods of time in their intended environments eliminates many potentially useful refractory materials. The reordering of crystalline phases, which commonly occurs in ceramic materials subjected to these environments, impairs the desired physical and chemical properties. The result is a degraded structure which is no longer appropriate for its intended use.
It is known in the art that the inclusion of rare earth oxides and iron oxides to compositions consisting essentially of aluminum titanate, provides the body with sintering aids and further increases stabilization to high temperature degradation. It has not been conclusively determined how the stabilization is effected, although it is known that Fe.sub.2 TiO.sub.5 is in solid solution with Al.sub.2 TiO.sub.5. The solid solution is effected during firing, and is facilitated at high temperatures, above about 1400.degree. C. The role the rare earth oxide plays is to affect the grain growth behavior, thus adding mechanical strength.
It has been found with the present invention that the addition of surprisingly large amounts of Fe.sub.2 O.sub.3 may be incorporated in the Al.sub.2 TiO.sub.5 matrix. This combination may then be subsequently extruded and sintered to form a honeycomb structure. The resultant structure produces a thermally durable product with improved physical properties, unknown to the prior art.
In U.S. Pat. No. 4,483,944 (the '944 patent), an aluminum titanate-mullite ceramic composition is disclosed which includes 0.5 to 5% iron oxide and 0.5 to 5% rare earth oxides. In the '944 patent it was disclosed that 0.5 to 5% iron oxide and 0.5 to 5% rare earth metal oxides will most desirably be present to serve as a sintering aid and to inhibit the decomposition of Al.sub.2 O.sub.3.TiO.sub.2 crystals when exposed to high temperatures.
U.S. Pat. No. 4,327,188 (the '188 patent) discloses the use of aluminum titanate with the rare earth and iron oxide additives. The '188 patent discloses that there is a disadvantage to adding more than 2 weight percent of the specific additive due to an increase in the thermal expansion and a decrease in the melting point.
That there is a trade-off between the thermal properties and long life of aluminum titanate bodies has been known to the art. A remaining difficulty in the art is to insure stabilization of the additive-laden aluminum titanate body. A goal has been to find a body that can withstand temperatures in excess of 1400.degree. C. and maintain the crystalline integrity at lower temperatures. The body should be able to withstand, without significant decomposition, extended use at temperatures between approximately 1000.degree. C. and 1300.degree. C. This property is important since it is well known to those skilled in the art that Al.sub.2 O.sub.3.TiO.sub.2 will decompose into corundum and rutile when exposed to temperatures within the above cited temperature range. To guard against this decomposition, it is necessary to thermally stabilize the aluminum titanate phase.
Therefore, the primary objective of the present invention was to develop an aluminum titanate-containing body which exhibits high mechanical strength, a low linear coefficient of thermal expansion, is capable of extended use at temperatures in the vicinity of 1400.degree. C., while maintaining crystalline integrity after very prolonged exposure to temperatures within the range of 1000.degree.-1300.degree. C. Additionally, the body is capable of repeated cyclings in the temperature range between room temperature and well over 1000.degree. C. without significant change in dimensional integrity.